feat(android): add local PDF intake

[alirezasafaei-dev]

What changed

• adds a local-only PDF picker entry point to the Android documents app
• accepts and displays only PDF metadata (name, size, MIME type)
• preserves a prior valid selection on picker cancellation or invalid metadata
• keeps Android Uri handling in the app layer; the Home model has no URI or document bytes

Privacy and safety

• no INTERNET or storage permission was added
• no selected document is copied, persisted, or uploaded

Validation

• verifyArchitecture verifyAndroidPrivacy test lint :apps:documents:assembleDebug :apps:documents:assembleRelease (JDK 17) passed
• debug APK installed and launched on API 36 emulator; RTL home screen and document-picker launch were observed
• concrete PDF selection remains unverified because the emulator's document provider was empty